• Aaptotoichus challengeri Holbourn & Kaminski, 1995 †: 0.2 - 2.0 mm
• Aaptotoichus clavellatus (Loeblich & Tappan, 1946) †: 0.2 - 2.0 mm
• Aaptotoichus mutinensis Haig & McCartain, 2010 †: 0.2 - 2.0 mm
• Aaptotoichus quinnaniensis Haig, 2003 †: 0.2 - 2.0 mm
• Aaptotoichus vallis (Trifonova, 1967) †: 0.2 - 2.0 mm
• Abadehella biconvexa Okimura & Ishii, 1975 †: 0.2 - 2.0 mm
• Abadehella coniformis Okimura & Ishii, 1975 †: 0.2 - 2.0 mm
• Abadehella grandis Pronina-Nestell, 2001 †: 0.2 - 2.0 mm
• Abadehella iwaiensis Kobayashi, 1997 †: 0.2 - 2.0 mm
• Abadehella koryakiensis Daydov in Davydov et al., 1996 †: 0.2 - 2.0 mm
• Abadehella mengcunensis Wang, 2002 †: 0.2 - 2.0 mm
• Abadehella minima Wang in Zhao et al., 1981 †: 0.2 - 2.0 mm
• Abadehella tarazi Okimura & Ishii, 1975 †: 0.2 - 2.0 mm
• Abadehellopsis longlinensis (Lin, Li & Sun, 1990) †: 0.2 - 2.0 mm
• Abadehellopsis pauciseptata Vachard in Krainer, Vachard & Schaffhauser, 2019 †: 0.2 - 2.0 mm
• Abadehellopsis protea (Cushman & Waters, 1928) †: 0.2 - 2.0 mm
• Abadehellopsis regularis (Lin, Li & Sun, 1990) †: 0.2 - 2.0 mm
• Abadehellopsis regularisaeformis (Lin, Li & Sun, 1990) †: 0.2 - 2.0 mm
• Abathomphalus asteroidalis Salaj, 1983 †: 0.2 - 2.0 mm
• Abathomphalus intermedius (Bolli, 1951) †: 0.2 - 2.0 mm
• Abathomphalus mayaroensis (Bolli, 1951) †: 0.2 - 2.0 mm
• Abathomphalus salajensis Solakius, 1983 †: 0.2 - 2.0 mm
• Abatus (Pseudabatus) nimrodi (Koehler, 1911): 0.2 - 2.0 mm
• Abatus agassizii Mortensen, 1910: 0.2 - 2.0 mm
• Abatus beatriceae (Larrain, 1986): 0.2 - 2.0 mm
• Abatus cavernosus (Philippi, 1845): 0.2 - 2.0 mm
• Abatus cordatus (Verrill, 1876): 0.2 - 2.0 mm
• Abatus curvidens Mortensen, 1936: 0.2 - 2.0 mm
• Abatus ingens Koehler, 1926: 0.2 - 2.0 mm
• Abatus kieri McKinney, McNamara & Wiedman, 1988 †: 0.2 - 2.0 mm
• Abatus koehleri (Thiéry, 1909): 0.2 - 2.0 mm
• Abatus philippii Lovén, 1871: 0.2 - 2.0 mm
• Abatus shackletoni Koehler, 1911: 0.2 - 2.0 mm
• Abditodentrix pseudothalmanni (Boltovskoy & Guissani de Kahn, 1981): 0.2 - 2.0 mm
• Abdullaevia tosbulakensis Suleymanov, 1965 †: 0.2 - 2.0 mm
• Abelbolla boucheri Huang & Zhang, 2004: Meiobiota [0.2 - 2.0 mm]
• Abelbolla huanghaiensis Huang & Zhang, 2004: Meiobiota [0.2 - 2.0 mm]
• Abelbolla maemulensis Hong & Lee, 2014: Meiobiota [0.2 - 2.0 mm]
• Abelbolla major Hiang, Wang & Huang, 2015: Meiobiota [0.2 - 2.0 mm]
• Abelbolla warwicki Huang & Zhang, 2004: Meiobiota [0.2 - 2.0 mm]
• Aberisphaera gambanica Wan, 1991 †: 0.2 - 2.0 mm
• Abertella aberti (Conrad, 1842) †: 0.2 - 2.0 mm
• Abertella cazonesensis (Kew in Dickerson & Kew, 1917) †: 0.2 - 2.0 mm
• Abertella dengleri Osborne & Ciampaglio, 2010 †: 0.2 - 2.0 mm
• Abertella gualichensis Martínez, Reichler & Mooi, 2005 †: 0.2 - 2.0 mm
• Abertella habanensis (Sanchez Roig, 1949) †: 0.2 - 2.0 mm
• Abertella kewi Durham, 1957 †: 0.2 - 2.0 mm
• Abertella miskellyi Kroh, Mooi, del Rió & Neumann, 2013 †: 0.2 - 2.0 mm
• Abertella palmeri Durham, 1957 †: 0.2 - 2.0 mm
• Abertella pirabensis (Marchesini Santos, 1958) †: 0.2 - 2.0 mm
• Ablabesmyia mala (Hutton, 1902): 0.2 - 2.0 mm
• Abnitocrella eberhardi Karanovic, 2006: 0.2 - 2.0 mm
• Abnitocrella halsei Karanovic, 2006: 0.2 - 2.0 mm
• Abnitocrella newmanensis Karanovic, 2006: 0.2 - 2.0 mm
• Aborjinia eulagiscae (Tchesunov & Spiridonov, 1985) Özdikmen, 2010: Meiobiota [0.2 - 2.0 mm]
• Aborjinia profunda Zograf, Semenchenko & Mordukhovich, 2024: Meiobiota [0.2 - 2.0 mm]
• Abortipedia indica Unnithan, 1962: 0.2 - 2.0 mm
• Abrardia catalaunica Bilotte, 1974 †: 0.2 - 2.0 mm
• Abrardia mosae (Hofker, 1955) †: 0.2 - 2.0 mm
• Abriolina mediterranea Luperto, 1964 †: 0.2 - 2.0 mm
• Abscondicola humesi Fiers, 1990: 0.2 - 2.0 mm
• Absonifibula bychowskyi Lawler & Overstreet, 1976: 0.2 - 2.0 mm
• Absonifibula estuarina Santos & Timi, 2009: 0.2 - 2.0 mm
• Absurdaster hungaricus Kroh, Lukeneder & Gallemí, 2014 †: 0.2 - 2.0 mm
• Absurdaster meriani (Ooster, 1865) †: 0.2 - 2.0 mm
• Absurdaster puezensis Kroh, Lukeneder & Gallemí, 2014 †: 0.2 - 2.0 mm
• Abuhammadina saidi Abd-Elshafy & Ibrahim, 1990 †: 0.2 - 2.0 mm
• Abyssalia foliformis Gooday & Holzmann, 2020: 0.2 - 2.0 mm
• Abyssalia sphaerica Gooday & Holzmann, 2020: 0.2 - 2.0 mm
• Abyssamina incisa Schnitker & Tjalsma, 1980 †: 0.2 - 2.0 mm
• Abyssamina poagi Schnitker & Tjalsma, 1980 †: 0.2 - 2.0 mm
• Abyssamina quadrata Schnitker & Tjalsma, 1980 †: 0.2 - 2.0 mm
• Abyssopedunculus brevis (Andres & Overstreet, 2013) Martin, Huston, Cutmore & Cribb, 2019: 0.2 - 2.0 mm
• Abyssotherma pacifica Brönnimann, Van Dover & Whittaker, 1989: 0.2 - 2.0 mm
• Acaenodera nautili Bray & Justine, 2011: 0.2 - 2.0 mm
• Acaenodera placophora Manter & Pritchard, 1960: 0.2 - 2.0 mm
• Acaenodera spinosior Etchegoin, Lanfranchi, Cremonte & Timi, 2006: 0.2 - 2.0 mm
• Acanthastrea brevis Milne Edwards & Haime, 1849: 0.2 - 2.0 mm
• Acanthastrea crenulata (Meneghini in d'Achiardi, 1868) †: 0.2 - 2.0 mm
• Acanthastrea dausi Chevalier, 1961 †: 0.2 - 2.0 mm
• Acanthastrea echinata (Dana, 1846): 0.2 - 2.0 mm
• Acanthastrea hemprichii (Ehrenberg, 1834): 0.2 - 2.0 mm
• Acanthastrea minuta Moll & Best, 1984: 0.2 - 2.0 mm
• Acanthastrea pachysepta (Chevalier, 1975): 0.2 - 2.0 mm
• Acanthastrea polygonalis Martin, 1880 †: 0.2 - 2.0 mm
• Acanthastrea rotundoflora Chevalier, 1975: 0.2 - 2.0 mm
• Acanthastrea subechinata Veron, 2000: 0.2 - 2.0 mm
• Acanthastrea termofurae Chevalier, 1961 †: 0.2 - 2.0 mm
• Acanthastrea turonensis Chevalier, 1961 †: 0.2 - 2.0 mm
• Acanthatrium alicatai Macy, 1940: 0.2 - 2.0 mm
• Acanthatrium eptesici Alicata, 1932: 0.2 - 2.0 mm
• Acanthatrium fugleri Marshall & Miller, 1979: 0.2 - 2.0 mm
• Acanthatrium hitaensis Koga, 1953: 0.2 - 2.0 mm
• Acanthatrium japonicum (Yamaguti, 1939) Skarbilovich, 1948: 0.2 - 2.0 mm
• Acanthatrium lunatum Williams, 1960: 0.2 - 2.0 mm
• Acanthatrium microcanthum Macy, 1940: 0.2 - 2.0 mm
• Acanthatrium molossidis Martin, 1934: 0.2 - 2.0 mm
• Acanthatrium nycteridis Faust, 1919: 0.2 - 2.0 mm
• Acanthatrium oregonense Macy, 1939: 0.2 - 2.0 mm
• Acanthatrium ovatum Yamaguti, 1939: 0.2 - 2.0 mm